Throttle fully open on start, unable to close

I hadn't started my bike up for about 10 days, while I was on vacation.
When I returned, with the bike on the center stand, I started the bike in neutral, and the bike throttle was seemingly stuck, fully open. The bike jumped away from me, and slid itself (upright, thankfully) about 5 feet across my garage. I may have stopped it sooner, but I struggled to kill the engine using the kill-switch cable attached to the Joe Hunt magneto that the previous owner installed. I'll be heading to Costco later today to get some new underwear. I let the bike sit for a while, checked to make sure everything was set the way I'd normally start the bike, and started the bike up again - same thing.

One thing I did differently than normal is that when the bike didn't start on the first few kicks, I blew into the carb's air breather a bit - a friend did this on the bike the last time it didn't start after having sat for a while. The bike has a Mikuni Carb. I doubt this was the issue, but I'd like to present all the info.

I'm new to mechanics and riding in general, but I'm trying to learn with and on this old beautiful machine (1974 Commando 850).
The throttle cable appears to have been loosened, significantly - the cable housing was separated complete from the metal tightening adapter. I'm not yet sure how to properly set the throttle cable, (I'd never touched it before, other than poking at it to ensure it was tight), but I'll look it up.

My question is - any ideas what (and/or what else) could have caused this? Throttle cable? Stuck throttle plate in the carb? Idiot move blowing into the aspirator?

Any help / ideas / guidance appreciated - thanks for reading.

Does the bike have a fiberglass tank or a lined steel tank? Dissolving rosin is a common reason for a stuck carburetor slide. Jim

Gosh I hope Jim is wrong about tank resin as can enter valve guides and rings too and implies too late to save tank. Carb springs should draw throttle closed with a fair snap as slide strike bottom. I'm a bit sensitive to surprise throttle sticking as ruined Ms Peel's un-expected spry engine performance. I found the cam chain jerked so tight it ruined the cam tensioner then later a cam lobe flattened. Likely a non issue in your case, but keep in back of mind monitoring-comparing to past performance to go by.

74 Commando's came out with steel tanks, unless someone replaced it with a F/G tank :?: :?:

Ashley

It is a metal tank.
Thanks for all of the replies thus far.

Has the metal tank been lined? Feel around inside the fill hole and see if it is bare metal or if it has a plastic like liner. Jim

It could be as simple as a cable that needs lubed. I would pull the tops of the carbs off, and see if the slides are free in the bores. You need to discover if it is throttle, cables or slides before a course of action can be determined.

Russ

PS-the loose cable sounds like stuck slides...might as well confirm it. Take the tank off, take the two screws out of the top of the carb and the whole assembly should slide out.

Hey gardenzombie,
Rule # 1 before starting the bike....
Twist the throttle a couple of times to make sure that it returns properly.
That is,... Being spring loaded, it should always return, fully, by it's self.
If it doesn't, you have reason for concern, and the obvious will happen.
Don't worry about flooding the engine, (by turning the throttle), they don't have accelerator pumps
like a car, so you can turn the hand piece many times if you desire.
The Carbs have springs inside that push the Slides back down and hence pull the cables and return
the hand piece back to the closed position.
Apparently something has hung up, either one or both of the Slides or the Cable or the Hand piece itself.
Yours sounds like the Slides...
Remove the Air filter and put your finger in the back of the Carbs where the filter was, theoretically you should
feel both slides go up an down as you twist the throttle, one or both might be stuck up (open).
If they move but get stuck occasionally, you will need to remove them and find the cause / curse.
Unless you have Allen head screws in the tops, you will need to remove the Carbs to work on them,..
All is not lost... Sometimes, because of wear inside the Carbs, and on the outer surface of the Slides, they can tend to
stick, as the Carb warps when being over tightened, so.. As you undue the nuts that hold the Carbs to the Manifolds, the Slide
may come loose and drop down, provided that the Cables aren't miss aligned or hung up (not sitting properly).
So.... there you go... Could be crap around the Slides, sticky cables, warped Carbs or all of the above.
Also, if the bike hasn't been ridden a lot recently, and is fitted with the original Air slides (chokes), check to make sure
they aren't sticking inside the Throttle slides.
Have fun, AC.

Thanks for the response.

The bizarre thing is that the throttle does return properly every time, and still does.
I'm using a Mikuni Carb, and following the throttle cable, it goes into the very top of the carb.
I can "hear" movement when I twist the throttle and listen at the carb, and if I hold the cable where it enters the carb, I can feel movement. That's not to say nothing is sticking inside...

I've added WD-40 everywhere the metal cable can actually be forced to be exposed (the cable can be pulled from where it enters the grip under the brake, and then again on either side of what I --believe-- is called the "ferrule"). I'm not sure yet how to attach an image here, or I'd show you what I'm talking about (though you probably know).

I'll give it a shot like this, and then if i crap myself again, I'll start at the carb end.

See below.

rvich said:

It could be as simple as a cable that needs lubed. I would pull the tops of the carbs off, and see if the slides are free in the bores. You need to discover if it is throttle, cables or slides before a course of action can be determined.

Russ

PS-the loose cable sounds like stuck slides...might as well confirm it. Take the tank off, take the two screws out of the top of the carb and the whole assembly should slide out.

Click to expand...

Russ - thanks for the response. To be honest, I'm not yet certain what the slides are, and I'm running a single Mikuni Carb. However, I'll start from the throttle cable where it connects to throttle on the handle-bars, working my way down to the carb, and then if I get there with no resolution, well - then i get to learn more about my carb - which is why I bought this bike in the first place.

Thanks again.

You need to check the needle for truness. Get it out and roll it on a flat surface. It takes a good eye to see it and deft touch to bring it back to true.

Attaching the needle and cables to these slides can be a real bitch (learning curve). It is easy to catch, bump and jam these soft needles out of true. I speak from my own experience in this regard. Let my mistake be your solution.

Just the slightest out of true will cause the needle to catch in the needle jet make it not come home consistently.

If the Throttle hand piece and Slide is snapping shut, or returning ok, then I
tend to think that perhaps the Idle adjustment screw has been accidently turned in, (clockwise)
to give a high idle speed.
Has someone been TUNING this bike...
Unscrew the Idle screw, (anti clockwise), it's the exposed, serrated screw /knob with a slot, on the left side of the Carb,
and should have a visible coil spring on it to hold it in place, not the one nearer the rear at a 45 deg. angle,
that's the Mixture screw.
Take hold of the Idle screw with two fingers and open the throttle and then let it snap shut, you should feel
the Slide inside the Carb tap on the Screw. Turn the screw out, anticlockwise, say 1/4 of a turn and do it again,
or just keep doing it (turning it out) untill you can't feel the Slide hitting the screw.
Theoretically, the Slide, or Carby is now closed right off and the engine would not idle.
When you say the engine was revving at full throttle, is that correct or a mild exageration, I guess you didn't have time to look
at the Tacho. lol.
Any how, turn that Idle down, then check to see if the Slide (the round Brass or Alloy thing inside the Carb that goes up and down) has
come right to the bottom of the Venturi (throat) with no visible gap under it (looking from the front). If the Slide is right down when the throttle is closed
then theoretically the engine won't run once the Choke has been turned off. It's a MIKUNI.
If it's not right down, you have a problem. Pull it out and see why, if it is right down...
Give it a kick, see what happens. if there's an improvement, reset the Idle when the engine warms up.
This is all a bit puzzleing because most bikes won't start on full throttle, when cold.
Hope you find something,..
AC.

It's even more interesting when the throttle sticks wide open when you've just accelerated to pass in traffic and want to pull in behind the next vehicle. I had that happen when riding the works "hack" 650SS. I was headed for the back of a bus at full throttle in second gear! Thank goodness for that kill buttn.

What caused my problem was with dual Amals, so it doesn't apply to your single Mikuni. I hadn't reakised the slides weren't interchangeable between carbs - they used to be hand-lapped to fit. The Plumstead factory had been running a single carb test program. I'd taken the twin carbs off and the bike was shipped to Plumstead for the tests and then shipped back. I reassembled the carbs and re-installed them, then set off for home without testing the bike. I'd never considered the slides wouldn't be intercahngeable.

Ok - problem solved!
The throttle cable length adjuster, (which, to inform newbies like me, is located about 4 inches from where the throttle cable enters into the throttle grip on the handlebars) was set such that the carb's piston valve was open half-way even when the throttle was in the fully re-coiled resting position.
To figure this out, we removed the air filter, then stuck a finger into the air intake apeture, and with the throttle at rest, could feel the piston valve was up half-way. Twisting the throttle caused, as expected, the piston valve to lift all the way up, and then letting go of the throttle caused the piston valve to drop back down only half-way, and half-way open was its resting place.

So, we spun the throttle cable adjuster all the way closed, and sure enough, as we did it, the piston valve dropped. Once we had it appropriately closed, we started the bike, and it was at maybe 2000RPM ... definitely serviceable, and the reason it was even that high was because of my own fiddling with the idle adjuster on the Mikuni carb. I've got it dropped down to 1200RPM now, and it's running smooth.

So, after all that, two things:
1. I really, truly appreciate all the comments and suggestions - especially those from AussieCombat and rvich.
2. I think I should be researching whether the proper throttle cable is installed, and/or which is the proper one to use.

Thanks again, all.

Sometimes the answer is staring us right in the face, but we just can't see it.
Great to be able to help.
Just check that you have enough free play on the cable, I go with a couple of mm., make sure that the engine doesn't rev up
when you turn the handlebars to full lock. (tight cable)
AC.

Hello, new to the forum... I have a Mk II with single Mikuni conversion. Two problems :
1) Engine running, I open the throttle a bit and the carb seems to stick open for a while, and then goes back to idle if I activate the twist grip once or twice. If I rev up a bit more it sticks there again and goes back to idle with a bit of wrist flicking. This happens on the road with a gear engaged (not nice) or if the bike is in neutral. I've opened the wrist grip and cable action seems normal. When engine is off I can hear the carb slide tapping down normally. This is new since all worked well until recently. The cable seating on top of carb is ok. Could it be the floats or carb parts varnished or else ?
2) The carb is attached to the rubber flange with a serflex screw collar. However tight, the carb comes out every now and then. It gets spit out of the flange when compression is high, and mostly when I use the choke. I can't get the collar tight enough to hold it in and I've been running with a rubber around the back of the carb to the two tank spigots. The carb slides out because there is not enough grip on the carb admission sleeve. Also there is a teflon sleeve inside the rubber flange, or around the admission pipe, as if my carb sleeve was not the right diameter to fit the flange. How can I get a better grip so that the carb doesn't get spit out ?

I have to say it Gilles, get a pair of Amals. Just kidding. If you like them make them work, but I haven't a clue. Only trouble with Amals is they wear out and that can be fixed. Even the idle with new slides, or repaired slides. Personal opinion.

Dave
69S

Sore a couple of Velo's with Mikunis on this weekend :? and a Holden with three Amal G.P.s .

" The carb slides out because there is not enough grip on the carb admission sleeve." probably just a transplant rejection . Give it medicinal Whiskey .